Introduction

Diesel engines are widely used for transport and power-supply and, therefore, occupational exposure to diesel exhaust is common. In 2012, the International Agency for Research on Cancer (IARC) classified diesel exhaust as carcinogenic to humans, mainly based on the increased lung cancer risk observed in epidemiological studies. In recent years, tightened emission regulations in the EU and other parts of the world have caused a significant evolution of diesel technologies, resulting in a change in the emission and composition of the exhaust. The present article reviews the health effects of diesel exhaust, exposure at workplaces and the means available to reduce the exposure.

Composition of diesel exhaust

Diesel exhaust is a complex mixture of gaseous and particulate components produced in the combustion of diesel fuels. The emission rate and composition of the exhaust depend, for example, on the type, operational condition and maintenance of the engine, on the composition and properties of the fuel, and on the exhaust after-treatment techniques in use[1]. The main gaseous components of diesel exhaust are carbon dioxide, oxygen, nitrogen, water vapour, nitrogen oxides and carbon monoxide. In addition, sulphur dioxide and various organic compounds, such as low-molecular-weight carbonyls, carboxylic acids, alkanes, alkenes and aromatics may be emitted in the gas phase[2].

In addition to the gases and vapours, diesel exhaust contains tiny particles which are formed in the combustion process and in the subsequent condensation of gas phase compounds. These particles are composed of elemental carbon, adsorbed organic compounds, sulphates, nitrates and trace amounts of other elements[1]. Diesel exhaust particles are respirable; approximately 90% of the particle mass exists in the fine size range (≤2.5 µm). Nanoscale particles (≤50 nm) make up to 90% of the particle number concentration[3]. Due to their small size, the particles may reach the pulmonary alveoli, the sensitive gas-exchange region of the lungs.

It has been reported that the use of biodiesel instead or as a blend with a fossil fuel may moderately reduce the emissions of particles, total hydrocarbons and carbon monoxide but at the same time, the emission of nitrogen oxides often increases[4]. In general, the biodiesel-derived exhaust gases contain less of genotoxic polycyclic aromatic hydrocarbons but more of irritative aldehydes and ketones.

Influence of emission regulations

In the past two decades, the exhaust emission standards for diesel engines have tightened significantly in the EU[5]. As an example, Figure 1 depicts the emission standards for heavy-duty diesel engines from 1992 to 2013. In 1992, the emission of diesel particles from these engines was regulated to 0.36 g/kWh, and that of nitrogen oxides to 8.0 g/kWh. In 2013, the corresponding values were 0.01 g/kWh for particles and 0.4 g/kWh for nitrogen oxides, representing a 20–36-fold reduction of the permissible emissions during the past 20 years. For off-road engines, the particle emission value was reduced to 0.025 g/kWh in 2011–2013. However, for the engines whose net power is below 37 kW, a higher particle emission, 0.6 g/kWh, is allowed, and the emissions from the smallest engines (<19 kW) are not regulated at all [5].

The tightened emission regulations in the EU and other parts of the world have stimulated a significant evolution of diesel engine and exhaust after-treatment technologies. The key developments have included electronic high pressure fuel injection systems, cooled exhaust gas recirculation, crankcase filtration, diesel oxidation catalysts (DOCs), and (wall-ﬂow) diesel particulate ﬁlters (DPFs)[6]. At the same time, the sulphur and aromatics content of diesel fuels have reduced. These changes have not only reduced the emissions, in particular the amounts of diesel particles and organic compounds, but it has also changed the composition of diesel exhaust. For example, elemental carbon, which is the main constituent of the particles produced by the traditional diesel engines, constitutes only a small proportion of the very small particle mass emitted by the new technology engines[7].

Health effects

Respiratory and cardiovascular effects

Exposure to diesel exhaust may evoke irritation of eyes, nose and throat and the experience of unpleasant smells[1]. A mild airway inflammatory response and increased airway resistance has been detected in volunteers exposed to relatively high levels of diesel exhaust (particles: 100–350 µg/m3; NO2: 0.2–1.6 ppm) for 1–2 hours (see [8], [9] and references therein). In addition to the respiratory effects, indications on cardiovascular effects, such as changes in the function of blood vessels, have been observed in the studies[9];[10]. Furthermore, epidemiological studies have associated ambient air pollution, especially particulate matter, with acute cardiac effects and development of chronic cardiovascular diseases[10]. However, the underlying mechanisms and relative importance of the different constituents of diesel exhaust to the observed effects are not fully understood.

In animal studies, exposure to diesel exhaust has been associated with increased response to allergens and decreased viral and bacterial clearance from the lungs[8]. In rats, long-term exposure to high levels of diesel exhaust has triggered inflammatory and histological changes in the lungs[1]. At very high exposure levels, lung tumours have also been detected.

Carcinogenicity of diesel exhaust

In June 2012, the International Agency for Research on Cancer (IARC) updated its evaluation on the carcinogenicity of diesel exhaust and re-classified diesel exhaust as carcinogenic to humans (Group 1)[11]. The update was mainly based on the recently published epidemiological studies on lung cancer risk related to diesel exhaust exposure among non-metal miners, railroad workers and workers in the trucking industry. Most notably, a large study among the US non-metal miners has showed an increased lung cancer risk with increasing exposure to diesel exhaust[12];[13]. The lung cancer risk among the workers with the highest exposure was nearly threefold in comparison with the lowest exposure group. There is also limited epidemiological evidence linking diesel exhaust exposure with increased incidence of bladder cancer. Since diesel exhaust is genotoxic to bacteria and mammalian cells, the IARC has concluded that diesel exhaust is likely to induce cancer through genotoxicity[11].

Health effects of new technology diesel engine exhaust

There is still very little data available on the health effects of new technology diesel engine exhaust. The few available animal studies indicate that application of advanced exhaust after-treatment techniques may significantly reduce the respiratory effects of diesel exhaust[14]. In an ongoing two-year inhalation study evaluating the exhausts from new technology diesel engines, mild inflammatory and histological changes were observed in the lungs of exposed rats after three months and one year of exposure at the highest dose tested (particles: 13 µg/m3; NO2: 3.6 ppm)[15]. No exposure-related genotoxic effects were found in rats or mice after three months of exposure.

Exposure at workplaces

Diesel engines are widely used for transport and power-supply, and are dominating power-sources for heavy-duty vehicles. The main advantages associated with diesel engines include their high efficiency, robustness and durability. In particular, the high energy efficiency makes the diesel engine an attractive alternative for many applications. In comparison with gasoline engine exhaust, diesel engine exhaust contains considerably less carbon monoxide which makes it possible to run diesel engines in enclosed worksites where gasoline engines cannot be used. In addition, diesel fuel is less volatile and flammable than gasoline, and is therefore considered a safer alternative.

Due to the wide use of diesel engines in trucks and other vehicles, as well as in forklifts, generators, tractors, excavators and other industrial, agricultural and construction equipment, exposure to diesel exhaust occurs at many workplaces. The exposed worker groups include mine and construction workers, warehouse workers, mechanics, emergency workers, professional drivers, and shipping and railroad workers. The exposure to diesel exhaust may also occur in agriculture, forestry, waste management, environmental remediation, and other industries where diesel-powered vehicles and tools are applied. In a study carried out in 15 EU countries in 1990–1993 (CAREX study), diesel exhaust was found to be the fourth most common carcinogenic agent in workplaces, with three million regularly exposed workers[16]. In 2009, exposure to diesel exhaust was assessed as the second most emerging chemical risk related to occupational safety and health by the EU-OSHA[17]. A 2015 report on Diesel exhaust emissions and quantitative risk assessment concluded that data on exposures amongst miners and the road haulage industry (Truckers) provided a useful basis for quantitative risk assessments of exposures in particular to older diesel engine exhaust[18].

In general, the highest levels of diesel exhaust have been measured at underground worksites, such as underground mines and tunnel construction sites[19]. Intermediate levels have been detected at (semi)enclosed above ground worksites, such as motor vehicle repair shops, warehouses and fire stations, and the lowest levels at outdoor worksites and in the cabins of diesel vehicles.

Exposure and risk assessment

According to the Chemical Agents Directive (98/24/EC), the employer must determine whether hazardous chemical agents are present in the workplace, and assess and control the risks they may pose to the safety and health of the workers. Exposure to diesel exhaust and other exhaust gases needs to be taken into consideration in the assessment. In order to carry out the assessment, the following information may be needed:

How many diesel engines are present at the workplace?

What are the type, age and condition of the engines? Are they regularly maintained?

How many people are potentially exposed to diesel exhaust? What is the level and duration of the exposure?

What control measures are in place? Are they working satisfactorily?

Have there been any ill-health complains related to the exposure?

The Health and Safety Executive in the UK has proposed simple criteria for a preliminary evaluation of the exposure levels and adequacy of the control measures for diesel exhaust exposure in the workplace (Table 1). If there is no visible haze, smoke or soot deposits in the workplace, and if the workers do not experience irritation of eyes, nose or respiratory tract, the control measures that are in place appear likely to be adequately protective[20]. In addition to signs of visible smoke, soot deposits or complains of irritancy, carbon dioxide levels exceeding 1000 ppm (8 hours time weighted average) may indicate an inadequate, faulty or poorly maintained control system[21]. When more accurate information on the exposure levels is needed, exposure measurements and comparison of the results with the available occupational exposure limit values may be carried out.

Table 1: Criteria for assessing the adequacy of control measures against diesel exhaust exposure

Exposure measurements

Respirable particles, elemental carbon, and nitrogen oxides are commonly used indicators of diesel exhaust exposure. A challenge of respirable particle measurements is the difficulty to differentiate diesel exhaust particles from other particles and dusts at the worksite. Elemental carbon is a more specific indicator for diesel exhaust. The variable proportion of elemental carbon in diesel exhaust particles may, however, cause uncertainty in the interpretation of the results. For a more comprehensive perspective on the exposure, determination of more than one exhaust component is recommended. For example, respirable particles and/or elemental carbon, indicators for the particulate components, may be measured together with nitrogen dioxide, which is an indicator for the gas phase components. Table 2 gives an example of measured levels of particulate constituents and nitrogen dioxide at different workplaces where exposure to diesel exhaust occur[22].

Occupational exposure limit values

Since diesel exhaust is a complex mixture of gaseous and particulate components, the occupational exposure limit values (OELs) of the single components do not necessarily protect from the health effects of the whole exhaust. Although the levels of the single components of the exhaust usually are below their OELs, health effects may still occur due to the combined exposure. There is, therefore, a need for limit values for diesel exhaust as a whole. However, only a few countries have determined OELs for diesel exhaust (Table 3).

In Austria, technical guidance concentrations (TRK values) for diesel exhaust particles are given separately for underground mining and for other workplaces[23]. The values are based on technical feasibility and do not consider health effects of diesel exhaust. In Switzerland, elemental carbon in the respirable fraction of diesel exhaust particles is applied as the indicator substance for diesel exhaust exposure[24]. In Sweden, limit values for exhaust gases, including diesel exhaust, are given as nitrogen dioxide and carbon monoxide[25]. Nitrogen dioxide, in particular, is considered as a suitable indicator for diesel exhaust exposure. The values are based on the irritative and inflammatory effects of short-term exposure to exhaust gases [26]. In the United States, a limit value of 20 µg/m3, measured as elemental carbon (8 hours time weighted average), was withdrawn in 2003. The reason for the withdrawal was the outdated background documentation of the value[27]. With respect to underground mining, a limit value of 160 µg/m3 of total carbon still applies[28].

The Nordic Expert Group for Criteria Documentation of Health Risks from Chemicals (NEG), in co-operation with the Dutch Expert Committee on Occupational Safety (DECOS), has prepared a criteria document on the health effects of diesel exhaust. It is intended that this document may be used as a scientific basis for setting of health-based occupational exposure limit values for diesel exhaust. In the future, the work will result in updated reference values and also recommendations for the exposure assessment. A key finding was the observation that "the significant reduction of the DEP mass concentration in the new technology diesel engine exhaust is expected to reduce the lung cancer risk (per kWh)." However, the report also noted that "it is important to take into account that the transition from “old” to “new” technology diesel engines is expected to take a long time"[29].

Exposure control measures

Prevention and control of diesel exhaust exposure at workplace adhere to the general hierarchy of controls applied to control occupational exposure. Where appropriate, the substitution of diesel engines with alternative power sources, such as electric or natural gas fuelled engines should be considered. A significant reduction of exhaust emission may also be achieved by replacing older diesel engines with new engines fulfilling the tighter emission regulations of today. Alternatively, older diesel engines may be retro-fitted with new exhaust after-treatment devices, such as diesel oxidation catalyst (DOC) or diesel particulate filter (DPF)[31]. Regular maintenance of the engines is of great importance, since there tend to be high emissions from badly maintained engines.

When diesel engines are run indoors, efficient natural or mechanical ventilation need to be provided to remove the exhaust and to provide sufficiently fresh replacement air. Where appropriate, enclosing the tailpipe with a flexible hose extraction system vented outside is an effective measure for removing engine exhaust[20]. Unnecessary running or idling of diesel engines should always be avoided, especially indoors. The examples of exposure control measures for specific workplaces are listed in Table 4.

Diesel exhaust is best controlled at source or by efficient general ventilation. Respiratory protective equipment should only be used as a last resort if the exposure cannot be sufficiently controlled by other means. If respiratory protective equipment needs to be used, the equipment chosen should be suitable for protecting against the particles, inorganic gases and organic vapours in the exhaust, preferably an air-supplying respirator.

Concluding remarks

There is increasing evidence of the respiratory and cardiovascular health effects of the exhaust from older technology diesel engines. In particular, the updated carcinogenicity classification by the IARC emphasizes the need to take diesel exhaust exposure into close consideration in the risk assessment and planning of control measures at a workplace. Special attention should be paid to diesel exhaust exposure in mines, underground construction sites and other enclosed worksites where high levels of exhaust may easily build up. The key measures for exposure control include substitution of diesel engines with lower emission engines, regular maintenance of the engines, and efficient ventilation systems.